



United States ?aten-t lRGCESSFOR THE PURIFICATION OF AROMATIC CARBOXYLIC ACIDS Franz Broich and Gunthard Hotfrnann, Marl in Westphalia, Germany, assignors to Chemische Werke Huls Akfiengesellschaft, a corporation of Germany No Drawing.- Application February 15, 1954, Serial No. 410,434-

Claims priority, application Germany August 17, 1953 2 Claims. (Cl. 260-525) It is known how to produce aromatic monoand polycarboxylic acids from the corresponding aralkyl hydrocarbons by oxidation. The air oxidation of such aralkyl hydrocarbons comes to an end relatively quickly, especially when there are several side chains and side chains having more than one C atom, even in the presence of catalysts and with the use of elevated pressure and elevated temperature. The yield of aromatic monoor poly-carboxylic acids remains small, while the principal products of the reaction are carbonyl and hydroxyl compounds, and as secondary products, esters and resins.

The known oxidation of aralkyl hydrocarbons to the corresponding aromatic carboxylic acids by means of nitric acid leads to the desired acids in technically acceptable yields, especially when operating at elevated temperature and under pressure. A considerable disadvantage of this method is, however, that the occurrence of nitrogen-bearing by-products cannot be avoided even when highly dilute nitric acid is used. These nitrogen compounds can be removed from the end pro ucts only by complicated and expensive operations, but their removal is absolutely necessary for example, for dicarboxylic acids which are to be used as softeners or for synthetic fibers. Another disadvantage of nitric acid oxidation is that the end product separates from the aqueous solution during the oxidation and to some extent occludes the starting material and intermediate products of oxidation and thus prevents further action of the nitric acid thereon. As a result of these phenomena, the carboxylic acids are colored more or less intensively and are not of a high degree of purity in the acid content.

These disadvantages of oxidation with air and with nitric acid are avoided in the known oxidation with alkaline solution of potassium permanganate and with neutral solution of alkali metal bichromate. In both cases the acids are present at the end of the oxidation as watersoluble alkali metal salts and can easily be separated from the inorganic reaction products and from non-acid contamiuations. Of the two processes, especially the oxidation in aqueous solution with alkali metal bichromate, under pressure and at elevated temperature has found application on an industrial scale for the oxidation of for example toluene. The disadvantage of the oxidation of aralkyl hydrocarbons with alkali metal bichromates in aqueous solution is the large amount of chromium oxide (C1'203) produced, which for reasons of economy must either be recovered as pigment or reconverted into bichromate for reuse. Since one mole of chromium oxide is produced per methylene group oxidized, the oxidation of aralkyl hydrocarbons having several or relatively long side chains is technically unattractive because of the large amounts of chromium oxide to be handled.

It has now been found that aromatic carboxylic acids can be produced much more advantageously from the respective aralkyl hydrocarbons by oxidation at elevated temperatures if the mixtures obtainable by treatment with nitric acid are re-oxidized or the oxidation is completed Patented May 14, 1957 with aqueousalkalimeta-l-bichromate. It may be advantageous to treat: the aralk-yl hydrocarbon with oxygen or oxygen-bearing gases before the treatment with nitric acid, until no further oxygen is' absorbed, possibly in the presence of oxygen carriers such as naphthenates, etc. Suitable aralkyl hydrocarbons are, for example xylene, diethyl benzene, diisopropyl benzene, cymene, etc. means of the nitric acid oxidation, which may occur under' pressure if necessary, it is possible to transform aralkyl hydrocarbonshaviugseveral and/or relatively longiside chains into aromatic monoor polycarboxylic acids. The mixtures obtained, which contain besides varying quantities of nitrogen-bearing by-products and discoloring and resinifying intermediate stages, are, after separation of the nitric acid, neutralized preferably with an alkali metal hydroxide and heated in aqueous solution with the required quantity of alkali metal bichromate, which can easily be determined by a preliminary experi ment, under pressure to temperatures of between 180 and 300 C., preferably to 250-280 C. The nitric acid oxidation as well as the treatment with alkali metal bichrornate can be carried out continuously. Or the mixture obtained by oxidation with nitric acid can, after removal of the nitric acid be treated with the alkali metal bichromate solution. Upon completion of the reaction, the reaction mixture is cooled, removed from the separated chromium oxide, and acidified. The aromatic monoor polycarboxylic acids thus obtained are pure white and no longer contain nitrogen in analytically detectable quantities.

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Example To 1620 parts by weight of p-diisopropyl benzene are added 1.6 parts by weight of cobalt naphthenate containing 10% Co and the mixture is treated at 130 C. in a reaction tower with a mixture of 10 parts by volume of air and 6 parts by volume of oxygen, thorough mixing of the gas and the hydrocarbon being efiected by means of a porous ceramic plate. The rate of gas feed is so adjusted that the oxygen is consumed practically 100%, the temperature being raised to 170175 C. during the oxidation. After about 30 hours of operation with an oxygen consumption of about 1.5 mole per mole of hydrocarbon, the absorption of additional oxygen becomes so slow that this oxidation treatment is stopped. At that moment the yield, referred to the oxygen consumed, is about 16.7%. 1665 parts by weight of a dark brown oil of the following composition are obtained:

This oil is treated in a vessel equipped with an intensive agitator and a reflux condenser, at -105 C. by the gradual addition of ten times the quantity by weight of a 25-30% nitric acid. The nitric acid concentration in the reaction mixture is maintained by the addition as required of a more highly concentrated nitric acid. After about -100 hours there is no further consumption of nitric acid. The total consumption is then about 5 parts by weight of nitric acid per 1 part by weight of the oil. The resulting reaction mixture is filtered and washed with boiling water. There are obtained 1255 parts by weight of crude terephthalic acid in the form of a brown-yellow powder. The acid number is 614, corresponding to a purity of 91%. The nitrogen content is 0.62%.

This 1255 parts by weight of crude terephthalic acid is neutralized with 25% sodium hydroxide solution and heated in an autoclave with 450 parts by weight of NazCrzOmZI-hO for 24 hours at 280 C. The pressure rises to about 75 atm. After cooling, the product is filtered from the separated chromium oxide and acidified with hydrochloric acid. There are obtained 1218 parts by weight of pure white terephthalic acid of an acid number of 672. The melting point of the dimethyl ester is 140.8-Q. Nitrogen is no longerdetectable analytically. Referred to the p-diisopropyl benzene used, the yield of terephthalic acid is 73.4%

We claim:

1. Process for the purification of an aromatic carboxylic acid obtained by oxidizing an aralkyl hydrocarbon with nitric acid at elevated temperature which comprises treating the resulting reaction product containing aromatic carboxylic acid with an aqueous solution of an alkali metal bichromate at an elevated temperature between 180 and 300 C.

2. Process for the purification of an aromatic carboxylic acid obtained by oxidizing an aralkyl hydrocarbon with an oxygen containing gas at elevated temperature and oxidizing the resulting product with nitric acid at elevated temperature, which comprises treating the resulting reaction product containing aromatic carboxylic acid with an aqueous solution of an alkali metal bichromate at an elevated temperature between 180 and 300 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,998,925 Demant Apr. 23, 1935 FOREIGN PATENTS 63,987 Netherlands Aug. 15, 1949 

1. PROCESS FOR THE PURIFICATION OF AN AROMATIC CARBOX YLIC ACID OBTAINED BY OXIDIZING AN ARALKYL HYDROCARBON WITH NITRIC ACID AT ELEVATED TEMPERATURE WHICH COMPRISES TREATING THE RESULTING REACTION PRODUCT CONTAINING AROMATIC CARBOXYLIC ACID WITH AN AQUEOUS SOLUTION OF AN ALKALI METAL BICHROMATE AT AN ELEVATED TEMPERATURE BETWEEN 180 AND 300*C. 